- multiple help sheets with current active window(process) auto detection

While in firefox, super key pops up ff key shortcuts.
While in terminal, super key pops up some basic bash commands or readline keys. Even better, detect current process at that moment.

I don't think unity support most apps cheetsheets. But, like bash-completion, if we can get some base infra for adding sheets and detecting process, then users or application developers can add their own.
Users can get quick keyboard help if they remember 'Super' key. Simple is function!